I want to install window air conditioner and want to see how to
seal around air conditoner that doesn't have curtain or accordian to add around air conditioner. I must seal to keep water out.
Thanks for helping.

a few years ago a customer asked me to find a way to do that for them. i bought a sheet of plexiglass at home depot, cut it to size and used some leftover weather stripping and they were very happy with it.

The following video portrays how this project can be done using a piece of 3/4" plywood (preferably pressure-treated plywood); a measurement of that part of the external exhaust section of the AC that will fit thru the plywood has to be made with a pencil & a straight-edge & then the plywood is cut to that dimension; they use a circular saw in the video, which seems a little dangerous, a jig saw might be safer.

Appropriate fasteners (screws) are installed to strenghten the hold of the plywood around the perimeter of the window; often, 1 1/2 X 1 1/2 wood brackets are used (drilling pilot holes at several points) for more support.

For the window seal at the intersection of the double-hung window, a piece of 3/4" or 1" "pipe-wrap" insulation (plumbing supplies dept.)(available at HD/Lowe's), long enough to accomodate the diameter of the double-hung window, is forced between the upper & lower windows at the intersection to keep hot air & bugs out.

Use a caulking gun to seal any areas of the plywood where water may get in, then paint the plywood after a week or two when the treated wood plywood dries out sufficiently.